According to MicW's current instructions, you should set the device gain to Mid and the device units to FS. Then, set the sensitivity for the input channel according to the value that was written on the sheet that came with your i437L (e.g. 19.9 Pa/FS). (Make sure input channel units are set to Pa.)

We just tested this, today, with an iPhone X and XS Max, and the results agreed within 0.1 dB of a 94 dB field calibrator on both devices.

Keep in mind that the sensitivity given by MicW is valid for the Mid gain setting, only. If you want to measure higher or lower SPLs which would require Low or High gain, respectively, you would need to perform a calibration at the chosen gain.

It's true that there are no physical knobs for adjusting the Oscilloscope display.

Perhaps, you have already found that if you look in the Display tab of the drawer on the bottom side of the Oscilloscope window, you'll see the software controls for adjusting the amplitude (Vert) and time (Horz) scale of the scope.

Frequency measurements can be made with the Oscilloscope, using the dual data cursors, but it would be easier to measure frequencies with the FFT Analyzer. However, if you need very fine precision, it may be better to use the Oscilloscope.

If you are not seeing the option to select between FS and V for the device, that suggests that your MicW is either not connected properly, or that you have the built-in mic selected as the current input device. (FS stands for Full Scale, which is the maximum digital value that can be represented by the input hardware.)

If you think it will help, you are welcome to attach or send screenshots that illustrate the problem you are experiencing.

Unfortunately, the i437L presents itself as if it were a standard headset jack. This means that apps like SoundMeter cannot automatically distinguish between an i437L and a standard headset jack.

Your approach is correct (i.e. Device Units = FS, Channel Units = Pa, Gain = Mid). We do not know the precise input gain used by MicW when testing the sensitivity of the i437L, but using the Mid gain setting in SoundMeter seems to work well.

It's too bad MicW didn't take advantage of the opportunity to produce a digital microphone that presented itself uniquely to iOS. That would have allowed automatic loading of sensitivity information, similar to what SignalScope does with the Digiducer digital accelerometers.

Yes. With those settings, you'll get the average sound level over 20 seconds in each whole or 1/3 octave band, plus an overall (full bandwidth) average level. You can see the overall level by tapping the graph to turn on the cursor.

The Octave analyzer tool (RTA) supports both exponential (Lp) and linear (Leq) averaging of the signal in each whole or 1/3-octave frequency band. For exponential averaging, Fast, Slow, and Impulse time weightings are supported. For linear averaging, the average level is calculated over the entire duration of the measurement.

SignalScope's built-in documentation includes information on how to access the averaging options.

Why can't you use the output of the Duet to use the built-in test signals? Perhaps, a more detailed description of what you are trying to do, and how you would like to set it up, would help us zero in on a suitable solution.

Bluetooth mic input is typically sampled at 8 kHz. The weighting filters in the Meter and the bandpass filters in the Octave analyzer are designed to operate at multiples of 44.1 kHz and 48 kHz, which would explain the "Unsupported Sample Rate" message.

It seems that some time ago Apple changed the behavior of Bluetooth audio in iOS (at least in the Measurement Mode that SoundMeter uses). Since SoundMeter requires both input and output from the audio system, Bluetooth won't work unless you are using a Bluetooth headset (or hearing aids, in your case) with both input and output. Unfortunately, if you use a headset for input and output, then the sample rate will be restricted to 8 kHz (for both input and output).

This is a change from how Bluetooth used to work on iOS, when it was possible to use the built-in mic with Bluetooth output.

I'm not aware of any affordable sound level meter software for Windows.

You might consider an iPod touch as a less expensive alternative to the iPhone. The accuracy of your measurements will depend on the quality of your calibration, regardless of which device you use. You can get more reliable measurements with an external measurement microphone, like the MicW i436.